(1) Field of the Invention
The present invention relates to a novel diallyl terephthalate prepolymer and a method for preparing the same.
(2) Description of the Related Art
Diallyl phthalate resins prepared from diallyl phthalate (hereinafter referred to as "DAP") as a monomer have been widely used in molding materials, decorative plates, laminated plates and the like, and they are considered to be reliable in that their electrical characteristics do not deteriorate for a long period of time even under conditions of high temperature and high humidity.
Nevertheless, as the performance of machines and electrical parts is improved more and more, it is desired to develop resins retaining the present excellent features and additionally having improved heat resistance.
The monomer for the DAP resin has two double bonds, and therefore gelation of the DAP resin occurs inconveniently at the point of a low conversion, when polymerized in a usual manner. Thus, for the purpose of avoiding the gelation, the following procedure is now employed: The polymerization is stopped transiently prior to the gelation so as to take out the resulting polymer as a prepolymer, and a polymerization initiator and a filler are then added to the prepolymer to obtain a product which can be commercially used. Accordingly, an efficient production process for the prepolymer also is an important theme. The known methods for the prepolymer which have been heretofore suggested are not satisfactory yet, and therefore the development of the effective novel method is demanded.
In the case that diallyl terephthalate (hereinafter referred to as "DAT") which is conveniently symmetrical is used as a monomer, the improvement of heat resistance can be expected. However, it is known that three-dimensional network-cured resins of DAT are still poor in mechanical properties, and for example, they are brittle. For this reason, they have not been put into practice yet.
Heretofore, bulk polymerization and other polymerization methods using various solvents are known as techniques of preparing the prepolymer. However, with regard to the bulk polymerization not using any solvent, the iodine value of the prepolymer does not lower less than about 80, even when the polymerization is stopped just before the gelation, and in this case, the weight average molecular weight (hereinafter referred to as "Mw") of the prepolymer which is measured by a GPC method is extremely high. According to researches of the inventors of the present application, it has been found that when the three-dimensional network-cured resin is prepared by using the prepolymer having the excessively high iodine value and the high Mw, drawbacks such as rigid and brittle properties cannot be eliminated and at the time of molding, the viscosity of the material increases merely, and after all, physical properties of the cured resin are not improved at all.
A polymerization method using a solvent such as benzene or ethyl acetate is also known, but in this case, the iodine value lowers, but a molecular chain portion by a carbon-carbon bond of an allyl group in the monomer unit is still long, which means that the physical properties are not improved yet.
In Japanese Laid-open Patent Application No. 80409/1984, a synthetic method of the prepolymer is disclosed which comprises copolymerizing DAT and an aromatic hydrocarbon having at least one hydrogen atom at the benzyl position of toluene, xylene or the like to obtain a cured resin with excellent heat resistance and high mechanical strength. However, in order to obtain such a copolymer, it is required to spray the monomer, a polymerization initiator and the aromatic hydrocarbon through a special nozzle with stirring at a high speed, and therefore the polymerization and reaction devices must be contrived. In addition, this disclosed method requires a great deal of the initiator, and what is worse, it is not easy to reuse the monomer because of radical copolymerization.
Another polymerization can be conceived which uses a halogenated hydrocarbon or an aldehyde derivative having a relatively high chain transfer constant, but also in this case, a radical generated from a solvent is added to an allyl group, so that the so-called telomerization takes place and performance of the resulting polymer deteriorates.
A diallyl isophthalate resin is also industrially manufactured as a resin having high heat resistance, but the price of its monomer is too high to be practicable.
A method of using a specific monomer such as diallyl naphthalenedicarboxylate is suggested, but in this case, the monomer is so expensive that the method is not put into practice.
However, any methods mentioned above cannot improve the productivity of the prepolymer to a satisfactory extent.
As already known, it has been also tried to copolymerize the DAP monomer and another monomer, so that a copolymer with ethylene which is excellent in physical properties is obtained. However, also in the case, the gelation occurs during copolymerizing, because the DAP monomer has two double bonds. Therefore, the polymerization must be stopped prior to the gelation. Apparently, this method impairs productivity because of the problem of monomer circulation, though the physical properties are improved.
An unsaturated polyester mainly containing terephthalic acid is also known, but in this case, it is necessary to add maleic anhydride or fumaric acid as an unsaturated component. Therefore, the portion of the unsaturated component is poor in heat resistance, and a glass transition point (Tg) is not so high, either.
Japanese Patent Publication No. 13808/1970 discloses that a mixture of a diallyl ester of a dibasic acid, a trivalent or more-valent polyol and glycol is subjected to an ester exchange reaction in order to obtain a crosslinkable polyester of the polyol. However, according to the present inventors' pursuance, the polyester of the polyol which was based on phthalic acid or isophthalic acid had insufficient heat resistance, and the ester resin decomposed at a high temperature of 200.degree. C. or more.
In Japanese Laid-open Patent Application No. 142027/1976, an oligoester of a hydroxyl group at a molecular terminal is disclosed, but according to the present inventors' pursuance, the ester was in a liquid and viscous state, and handling also was difficult. In addition, the ester resin did not have sufficient heat resistance and decomposed at a high temperature of 200.degree. C. or more.